Search

Simple modification techniques affecting surface properties in a switchable manner are key to the development of smart materials. Layer-by-layer assembly of oppositely charged polyelectrolytes (poly(diallyldimethylammonium chloride) and (poly(styrene sulfonate)) over an electrode modified with a mesoporous silica thin film makes a platform with alternating amplification and sieving electrochemical properties defined by the terminating polyelectrolyte layer. The mesoporous silica thin films were formed via sol–gel processing in the presence of a soft template (Electrochemically Assisted Self-Assembly). The electrochemical properties of the designed platforms were studied in the presence of positively charged (Ru(NH3)63+), neutral (FcMeOH), and negatively charged (Fe(CN)64-) redox probes. Additionally, we provide proof-of-concept studies in which our platforms are applied to the electroanalytical screening of ascorbic acid and dopamine.

Vertically aligned mesoporous silica films can be generated by electrochemically assisted self-assembly (EASA) but the accurate control of their thickness is essentially restricted to the 100 nm range. Here we have developed a wet etching approach using dilute ammonium fluoride to gradually decrease the thickness of ferrocene-functionalized films down to 20 nm by increasing the etching time. The effectiveness of the process was followed by monitoring the decrease in the voltammetric response of ferrocene moieties that are progressively removed from the electrode surface upon silica etching. Film thickness variations have also been confirmed by profilometry and were consistent with the electrochemical measurements. Electron microscopy analyses indicated a uniform thickness decrease but also some loss in the integrity of the mesostructure after prolonged etching. Keywords: Oriented mesoporous silica membrane, Ferrocene functionalized thin film, Wet etching, Film thickness control, Electroactive organic-inorganic hybrid

Iron (II) tris(2,2′-bipyridine) complexes, [Fe(bpy)3]2+, have been synthesized and immobilized in organosulfonate-functionalized nanostructured silica thin films taking advantage of the stabilization of [Fe(H2O)6]2+ species by hydrogen bonds to the anionic sulfonate moieties grafted to the silica nanopores. In a first step, thiol-based silica films have been electrochemically generated on indium tin oxide (ITO) substrates by co-condensation of 3-mercaptopropyltrimethoxysilane (MPTMS) and tetraethoxysilane (TEOS). Secondly, the thiol function has been modified to sulfonate by chemical oxidation using hydrogen peroxide in acidic medium as an oxidizing agent. The immobilization of [Fe(bpy)3]2+ complexes has been performed in situ in two consecutive steps: (i) impregnation of the sulfonate functionalized silica films in an aqueous solution of iron (II) sulfate heptahydrate; (ii) dipping of the iron-containing mesostructures in a solution of bipyridine ligands in acetonitrile. The in situ formation of the [Fe(bpy)3]2+ complex is evidenced by its characteristic optical absorption spectrum, and elemental composition analysis using X-ray photoelectron spectroscopy. The measured optical and electrochemical properties of immobilized [Fe(bpy)3]2+ complexes are not altered by confinement in the nanostructured silica thin film.

Recent and potential applications of electrochromic materials include smart windows, optoelectronic devices, and energy conversion. In this study, we have incorporated bis(terpyridine) iron (II) complexes into vertically-oriented silica thin films deposited on indium-tin oxide (ITO) and their electrochromic behavior has been investigated. If 2,2′:6′,2″-terpyridine is commonly used as a ligand for forming metallo-supramolecular assemblies, with the objective to get metal-terpyridine complexes with multiple stable redox states, their simple and reliable arrangement into linear structures enabling effective electronic communication is however more challenging. We propose to overcome this difficulty by generating such complexes within vertical nanochannels on electrode. Terpyridine ligands were firstly immobilized by combining a click chemistry azide/alkyne approach with an electrochemically-assisted self-assembly (EASA) method used to grow an oriented mesoporous silica membrane bearing azide groups which were further derivatized with 4′-ethynyl-terpyridine ligands. The resulting terpyridine-functionalized films were consecutively dipped in an aqueous solution of Fe(BF4)2 and then in a solution of terpyridine in acetonitrile to form the bis(terpyridine) iron (II) complexes in situ. The electrochromic properties of the films functionalized at various levels were examined by monitoring the changes in their UV/Vis spectra upon electrochemical oxidation at controlled potential of +1.2 V vs. Ag/AgCl. Due to facile charge delocalization during the Fe2+ to Fe3+ redox process, the bis(terpyridine) iron (II) functionalized silica films exhibited electrochromic properties by changing from violet to non-colored using TBABF4 in acetonitrile as an electrolyte. The bis(terpyridine) iron(II) film experienced reversible electrochromic switching by applying +0.5 V in a reverse reduction electrochemical process. The Fe(tpy)2-functionalized silica thin films displayed a good contrast ratio (ΔT%) of 47% and relatively high coloration efficiency (CE) of about 245 cm2/C with a response time of coloring and bleaching of a few seconds (< 4 s).

Sol‐gel‐derived silica thin films generated onto electrode surfaces in the form of organic‐inorganic hybrid coatings or other composite layers have found tremendous interest for being used as platforms for the development of electrochemical sensors and biosensors. After a brief description of the strategies applied to prepare such materials, and their interest as electrode modifier, this review will summarize the major advances made so far with composite silica‐based films in electroanalysis. It will primarily focus on electrochemical sensors involving both non‐ordered composite films and vertically oriented mesoporous membranes, the biosensors exploiting the concept of sol‐gel bioencapsulation on electrode, the spectroelectrochemical sensors, and some others. [ABSTRACT FROM AUTHOR]